Ultraviolet (UV)-emitting phosphors are used in fluorescent lamp applications for skin tanning where both UVA and UVB radiation is needed. UVA is defined by the U.S. Food & Drug administration (FDA) as radiation from 320 nm to 400 nm and UVB is defined as radiation from 260 nm to 320 nm. UV-emitting phosphor is applied to the interior surface of the lamp envelope by a conventional fluorescent lamp manufacturing process. Typically, phosphor is suspended in a liquid medium together with a binder and one or more of a dispersing agent, an adhesive material, and a wetting agent. The phosphor suspension is then flowed through the tubes to coat the interior surface and then the tubes are heated to burn out the binder material. Fluorescent lamp manufacturers prefer water-based coating suspensions to organic-based suspensions because of environmental issues with the organic solvents. It is desirable to make the coating suspensions in large quantities to be held in tanks over a period of several days. However, in some instances, the “holdover” of the phosphor coating suspension can cause problems with lamp performance.
The most commonly used UVA-emitting phosphor is a lead-activated barium disilicate, BaSi2O5:Pb, which has its emission maximum at 351 nm. The main drawback of this phosphor is its low stability in water-based binder suspensions, which results in a low output maintenance in tanning lamps. This deficiency is often addressed by coating the surface of the barium disilicate phosphor with a protective material such as aluminum oxide. For example, OSRAM SYLVANIA Type GS201X, a commercial BaSi2O5:Pb phosphor, has good stability in water-based coating suspensions because of a conformal aluminum oxide coating that has been applied to the surface of each individual phosphor particle via a chemical vapor deposition (CVD) technique in a fluidized bed. Such coating techniques are described in U.S. Pat. Nos. 4,585,673, 4,710,674, 4,797,594, 4,825,124, and 5,223,341.
Europium-activated strontium borate, SrB4O7:Eu, is another common UVA-emitting phosphor, which has a peak UV emission at 371 nm. In order to extend the range of emitted UV wavelengths to 380 nm, many suntan lamp manufacturers incorporate SrB4O7:Eu as a second UVA-emitting phosphor in a blend with BaSi2O5:Pb. The phosphor blend ratio of SrB4O7:Eu generally ranges from 5 wt. % to 30 wt. %. Like the BaSi2O5:Pb phosphor, a drawback of using a SrB4O7:Eu phosphor in tanning lamps is its low holdover stability in water-based coating suspensions. The SrB4O7:Eu phosphor typically leaches borate anions and strontium cations into the aqueous coating solution during the holdover period. Therefore, its radiance maintenance in a fluorescent tanning lamp is poor, i.e., tanning lamps employing the SrB4O7:Eu phosphor from water-based suspensions exhibit a relatively large decrease in radiant output over time. In addition, the presence of borate anions in the aqueous phase can lead to weakening of the glass bulb after drying and baking of the coating. To obviate these disadvantages of using SrB4O7:Eu phosphor from water-based suspensions in lamps, a protective layer of coating on phosphor surface becomes necessary.
As described above, the prior art method of applying an aluminum oxide coating via CVD is effective in dealing with the holdover problem in the case of BaSi2O5:Pb phosphors. However, this solution requires relatively complex coating equipment and hazardous chemicals and may unacceptably increase the cost of the phosphor. Thus, it would be advantageous to have a simpler, more economical method for protecting SrB4O7:Eu phosphors and improving their stability in aqueous coating suspensions.